In a licensed-assisted access (Licensed-Assisted Access, LAA) system, an LAA node uses a channel resource by using a listen before talk (Listen Before Talk, LBT) rule. LBT is a carrier sense multiple access (Carrier Sense Multiple Access, CSMA) technology. That is, after detecting that a channel resource of an unlicensed spectrum is idle, the LAA node uses the channel resource of the unlicensed spectrum to transmit data. The LBT includes a load based equipment (Load Based Equipment, LBE) manner and a frame based equipment (Frame Based Equipment, FBE) manner. The LBE manner means that whether to contend for a channel and duration of occupying a channel are determined based on whether data needs to be currently sent and based on a to-be-sent data volume. The FBE manner means that a channel is preempted according to a frame structure and duration of occupying a channel is fixed.
After successfully preempting a channel in the LBE or FBE manner, an LAA node (for example, user equipment or a base station) occupies the channel to send data. A time during which the LAA node occupies the channel each time should not exceed maximum occupation duration. For example, in the FBE manner, the maximum occupation duration is 10 ms, and in the LBE manner, the maximum occupation duration is 13 ms. Therefore, after occupying the channel for a period of time, the LAA node needs to release the channel, and preempts a channel resource again when needing to use a channel.
In an existing LTE system, a synchronous hybrid automatic repeat request (Hybrid Automatic Repeat Request, HARQ) mode is used for uplink transmission between user equipment and a base station. As shown in FIG. 1, in the synchronous HARQ mode, the user equipment (User Equipment, UE) transmits a data packet in a subframe n for the first time, and the base station (for example, an evolved NodeB or an eNB) feeds back acknowledgement (Acknowledgement, ACK) or non-acknowledgement (Non-acknowledgement, NACK) information to the UE in a subframe n+4. If the UE feeds back the non-acknowledgement information (NACK), the UE needs to retransmit the data packet in a subframe n+8. However, due to a limitation of maximum occupation duration on an unlicensed carrier, the UE may successfully preempt only resources of a subframe n−3 to a subframe n+7. Therefore, as shown in FIG. 1, the UE cannot retransmit the data packet within this channel occupation period. The existing HARQ retransmission mechanism is not applicable to the LAA system. Consequently, feasibility and system performance of the LAA system are relatively poor.